The retinal photopigments of geckos behave in a unique manner in the temperature range from 5 degrees C to 30 degrees C. Dissolved in digitonin, these pigments bleach by an increase in temperature and regenerate by restoration of the original low temperature, the amount and rate of color change in both directions being dependent on the magnitude and rate of temperature change. It is proposed further to investigate this unusual thermal effect employing the visual pigment in solution and in situ within the visual cells. The possible molecular mechanisms behind this effect as well as the products of mild thermal bleaching will be studied, examing the following: effect of pH, effect of ionic strength, affect of various inhibitors, studying the circular dichroism. The present hypothesis for this mild thermal action is that of an easily reversible conformational change in opsin without an isomerization of the chromophore. If this is so, and if the pigment in situ also possesses thermal lability, then the way is open to determine whether or not isomerization is essential for excitation. This will be done by testing the effect, or lack of effect, of a pulse of heat on the isolated retina. The receptor potential will be employed as an index of excitation in this test.